Clean Energy

Solar

ORNL’s Solar Technologies program supports the U.S. Department of Energy (DOE) Solar Energy Technologies Office – SunShot Initiative goal to make solar energy cost-competitive with other forms of electricity by the end of the decade. To achieve this goal, ORNL performs innovative research and development activities in both Photovoltaics (PV) and Concentrating Solar Power (CSP) that can make the abundant solar energy resources in the U.S. more affordable and accessible for all Americans. Areas of research interest include novel earth-abundant and nano-engineered PV materials, bandgap engineering through multi-layering approach, innovative and non-planar solar cell architectures, smart inverters, and innovative approaches to mitigate efficiency losses due to obstructions.

Comparison of optical transmission between borosilicate glass slides with and without ORNL’s transparent anti-soiling coating. Inset shows the coating exhibits a water contact angle of 163 deg and rolling angle less than 5 deg.

One of the major issues associated with solar power efficiency is the reduction in incident radiation even before energy conversion begins. For solar reflectors and PV cells, surface obstruc­tions such as dust and sand accumulation are main contributors to this reduction. Studies have shown that reflectivity can be reduced by as high as 50% in 14 days for the mirrors employed in CSP plants. Similar findings have also been reported for PV systems efficiency. Presently, scheduled routine manual cleaning and brushing using deionized water is the dominant solution. In order to reduce cost and enhance efficiency, ORNL is developing a low-cost, transparent, anti-soiling (self-cleaning) coating that can be applied on most surfaces using conventional painting or spraying methods.

PV systems are frequently subjected to transient shadings that reduce the amount of incident sunlight. ORNL’s Cascaded H-Bridge Inverter is one potential solution to this problem.

Similarly, Solar PV systems that supply electricity to com­mer­cial and residential buildings are frequently subjected to transient shadings from rooftop structures or nearby trees. The panel areas that are affected by shading change with time of day, and conventional central inverters are not designed to provide maximum output under these conditions. While micro-inverters can provide optimum power due to independent maximum power point tracking (MPPT) controls, the significant number of required units makes this solution costly and less reliable. ORNL and University of Tennessee researchers are working to develop advanced “Cascaded H-Bridge Inverter” that uses multiple levels of MPPT control that adjusts to transient shading conditions, providing maximum value of power output available to the systems. In addition, modulation compensation is used to balance 3-phase grid current.